Some birds and some wild quadrupeds change their colour according to
the seasons of the year. The reason is that, as men change according
to their age, so the same thing happens to them according to the
season; for this makes a greater difference to them than the change of
age.
The more omnivorous animals are more vari-coloured to speak
generally, and this is what might be expected; thus bees are more
uniformly coloured than hornets and wasps. For if the food is
responsible for the change we should expect varied food to increase
the variety in the movements which cause the development and so in the
residual matter of the food, from which come into being hairs and
feathers and skins.
So much for colours and hairs.
7
As to the voice, it is deep in some animals, high in others, in
others again well-pitched and in due proportion between both extremes.
Again, in some it is loud, in others small, and it differs in
smoothness and roughness, flexibility and inflexibility. We must
inquire then into the causes of each of these distinctions.
We must suppose then that the same cause is responsible for high and
deep voices as for the change which they undergo in passing from youth
to age. The voice is higher in all other animals when younger, but
in cattle that of calves is deeper. We find the same thing also in the
male and female sexes; in the other kinds of animals the voice of
the female is higher than that of the male (this being especially
plain in man, for Nature has given this faculty to him in the
highest degree because he alone of animals makes use of speech and the
voice is the material of speech), but in cattle the opposite obtains,
for the voice of cows is deeper than that of bulls.
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